Cryptosporidium parvum has emerged from relative obscurity to recognition as an important gastrointestinal pathogen around the world. C. parvum is a leading cause of severe diarrheal illness, and not infrequently death among the immunocompromised, particularly AIDS patients. Unfortunately, control measures for cryptosporidiosis are currently underdeveloped. Chemotherapeutic approaches have so far proven unsuccessful and research efforts have been directed to intestinal cryptosporidiosis but not the extraintestinal forms that frequently affect the AIDS patient in advanced stages. The main objective of this project is to identify dimeric IgA monoclonal antibodies to this parasite that effectively control hepatobiliary cryptosporidiosis using murine models of infection. This approach will expose an effective immunotherapeutic mechanism to control infection in the immunocompromised host.